This invention relates to a lubricant composition miscible in hydrofluorocarbon and hydrochlorofluorocarbon refrigerants and a process for using such a lubricant composition in refrigerator compressors and refrigerator compressor built-in refrigeration apparatus. More particularly, the present invention relates to a lubricant composition which is less deteriorated when contacted with a refrigerant in refrigerators at relatively high temperatures and suitable for enhancing reliability of compressors and a process for using such a lubricant composition in refrigerator compressors and refrigeration apparatus containing refrigerator compressors therein.
Recently, the use of chlorofluorocarbons (CFC) is to be inhibited in order to prevent the ozone layer in the stratosphere from breakage. Such CFC's include R11 (trichloromonofluoromethane), R12 (dichlorodifluoromethane), R113, R114, R115, etc. For example, R12 which has been used in refrigeration cycle in refrigeration apparatus such as a refrigerator is an object to be inhibited. As replacing refrigerants, hydrochlorofluorocarbons (HCFC) containing one or more hydrogen atoms in the molecule such as R22, and R502 (azeotropic mixed refrigerants containing 48.8% by weight of R22 and 51.2% by weight of R115) are studied. The ozone layer breakage coefficient of R22 is 0.05 and that of R502 is 0.3 taking the ozone layer breakage coefficient of R12 as 1. In the future, a hydrofluorocarbon (HFC) containing no chlorine atom in the molecule will be employed eternally. A typical example of HFC is R134a which has the ozone layer breakage coefficient of 0, is nonflammable and has almost the same thermodynamic properties as R12. Thus, R134a can be used practically without changing remarkably the structures of refrigeration apparatus such as refrigerators and dehumidifiers and refrigerator compressors. However, R134a (CH.sub.2 FCF.sub.3) has very unique properties due to its special chemical structure, so that it is not miscible in refrigerating machine oils used in the refrigeration system of R12, e.g. mineral oils (naphthenic oils, paraffinic oils) and synthetic oils such as alkylbenzene. Thus, there arise various problems in that oil return becomes worse when the oil flows into the refrigeration system, resulting in retaining the oil in the heat exchanger to lower heat transfer ability, and in the worst case, the oil in the compressor becomes insufficient so as to cause seizing or abnormal wear on sliding portions due to poor lubrication. Therefore, development of refrigerating machine oils (or lubricants for refrigerants) suitable for refrigerant R134a is required.
As the lubricants having miscibility in R134a, there are proposed polyglycol oils (e.g. Japanese Patent Unexamined Publication Nos. 1-259093, 1-259094, 1-259095, etc.) and ester oils (e.g. Reito vol. 65, no. 756, pp 47-52 (Oct. 1990)). But the polyglycol oils have defects in that (i) hygroscopicity large (saturated water absorption rate: 10,000-30,000 ppm), (ii) electric insulating resistance is low (volume resistivity: 10.sup.12 .OMEGA..multidot.cm or less), and (iii) deterioration by oxidation is great. In the case of using a hermetic compressor wherein a motor which is a driving source is integrated, there is a fear of lowering insulating properties of motor coils and terminals, resulting in causing a problem in reliability. On the other hand, in the case of the ester oils, there can be obtained improvement in electric insulating properties (volume resistivity: 10.sup.12 .OMEGA..multidot.cm) and hygroscopicity (saturated water absorption rate: 1,000-5,000 ppm) compared with the polyglycol oils, but the hygroscopicity is still larger than the saturated water absorption rate of 50-100 ppm of mineral oils and alkylbenzenes, which are refrigerating machine oils used in the R12 system and hydrolysis of ester oils is inevitable from their molecular structures. Thus, when an ester oil having a high water content is used as a refrigerating machine oil, the ester oil is hydrolyzed to form an organic acid near sliding portions of the compressor wherein the temperature becomes highest in the refrigeration system, resulting in increasing the total acid value of the refrigerating machine oil. As a result, there take place corrosion or damage of sliding materials of the compressor, formation of copper plating, reduction of moving of the sliding portions (e.g. abnormal increasing of the torque to rotate a shaft) due to formation of a viscous metallic soap, blocking of capillary tube in the refrigeration cycle. Thus, long period reliability of compressors and refrigeration apparatus is remarkably damaged. Improvement of this is desired.
On the other hand, addition of epoxy compounds to refrigerating machine oils is also proposed. For example, addition of an alkylene oxide compound to mineral oils or synthetic oils to improve thermal stability of CFCs is proposed (Japanese Patent Unexamined Publication No. 57-10694). In this reference, it is stated that styrene oxide and phenyl glycidyl ether cannot be used due to high toxicity and extremely strong skin irritation, epoxidized soy bean oil is low in thermal stability and shows a tendency to form a sludge freed from the oil in the co-presence of CFC refrigerator, and contrary to these compounds, special alkylene oxide compounds are low in toxicity and able to improve thermal stability of refrigerators. But refrigerants used in working examples are only R12, the use of which is to be inhibited. There is no description as to replacing refrigerants or as to what effects can be expected by using the alkylene oxide compounds. In fact, effects of the alkylene oxide compounds against R134a are insufficient.
Further, there is proposed a refrigerating machine oil obtained by adding a glycidyl ester of a straight-chain or side-chain saturated fatty acid having 8 to 18 carbon atoms or straight-chain unsaturated fatty acid having 14 to 18 carbon atoms to a polyhydric alcohol ester or a mixture of polyhydric alcohol ester and a mineral oil or synthetic oil (Japanese Patent Unexamined Publication No. 62-292895). In this reference, it is stated that the addition of glycidyl ethers such as phenyl glycidyl ether, epoxidized fatty acid esters, epoxidized vegetable oils, glycidyl acrylate, etc. is not preferable due to strong skin irritation in some cases, low effects, or the like, and addition of long-chain alcohol glycidyl ether (having one epoxy group) undesirably causes a phenomenon of copper plating, corrosion of copper, etc. But even if an effective fatty acid glycidyl ester is used under severe conditions of high temperatures, an acidic substance formation reaction due to hydrolysis of fatty acid glycidyl ester and an acid catching reaction by the glycidyl group proceed simultaneously, so that an acid catching effect as a whole is reduced. Thus, metallic parts of the compressor are damaged due to corrosion and wear with non-caught acidic substance, resulting in blocking of passage of refrigerant in a refrigeration apparatus by metallic reaction product of fatty acid and causing a problem of lowering performance due to lack of cooling.
Further, in the refrigeration cycle using R22 or R502 as a refrigerant and a mineral oil or alkylbenzene as a refrigerating machine oil, copper plating takes place on the surface of sliding portions of compressor, resulting in providing a problem of reliability of compressor.